1. Field of the Invention
This invention relates to the field of shimming of electromagnets. More specifically, the present invention comprises a new magnetic shimming configuration having optimized turn geometry and electrical circuitry.
2. Description of the Related Art
High-field magnets are widely used for many different applications including fundamental research, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) and Fourier-Transform Ion Cyclotron Resonance (FT-ICR). A typical high-field magnet creates a magnetic field that is not completely uniform. That is, if one plotted the strength of the magnetic field in physical space, one would see many peaks and valleys. In many applications, it is desirable to have magnetic field that is more nearly homogenous across a specific region. High-field magnets commonly employ resistive and superconductive shims to shape the magnetic field in order to make the field more uniform. For example, MRI magnet systems employ magnetic shimming to create the field homogeneity required to create detailed images.
The term “shim”—which is well known to those skilled in the art—refers to a magnetic analogy to a physical shim used to level and stabilize a mechanical device. In the context of an electromagnet, a “shim” is a passive or active conductor which alters the magnetic field. As an example, if there is a dip in the field strength in a particular region (and generally in a particular orientation), a shim can be placed to increase the field strength in that region. Likewise, if there is a spike in the field strength in a particular region, a shim can be placed to reduce the field strength in that region.
Shims can be passive or active. A passive is typically just a volume of ferromagnetic material placed in a desired location (such as an iron ring placed around the central bore of an electromagnet). A passive shim does not conduct externally applied electrical current, though it may of course conduct some induced current. An active shim, on the other hand, does conduct externally applied electrical current. This current may be varied in order to vary the corrective magnetic field created by the shim.
The use of both passive and active shims is well known in the art. Because the present invention concerns active shims, the previously existing techniques regarding active shims will be discussed in more detail. FIGS. 1-7 graphically depict commonly used active shims.
The lower view of FIG. 1 shows a conventional coordinate system using x, y, and z axes. The +x, +y, and +z axes are shown and labeled. Those skilled in the art will know that each axis has a negative counterpart extending in the opposite direction. Thus, the −z axis extends downward with respect to the orientation shown in the view. The −x axis extends into the page to the right. The −y axis extends into the page and to the left. The coordinate axes are conventionally used to describe positions and directions. As an example, the +z direction would be upward with respect to the orientation of FIG. 1, while the −z direction would be downward.
Electromagnets typically assume a cylindrical configuration, with the windings wrapping around central axis 34. The z-axis of the coordinate system conventionally lies along central axis 34. Electromagnets are typically symmetric about a mid-plane. The x-axis of the coordinate system is placed on mid-plane 32 (It is obviously also orthogonal to the z-axis). The y-axis is orthogonal to both the z-axis and the x-axis. It also lies on the magnet's mid-plane. The principles involved in the present invention are scalable. Thus, the physical location of each of the shims is expressed in terms of ratios rather than absolute dimensions. The symbol “a” represents the radius of a particular current loop. Thus, the azimuthal plot of FIG. 1 shows the position in terms of the ratio z/a, rather than an absolute dimension. For example, a particular prior art Z Shim has one turn located at a z/a position of +0.8, and one turn located as a z/a position of −0.8. If the radius “a” of the turns is 0.100 m, then the position of the two turns in the z direction would be +0.08 m and −0.08 m.
Large prior art electromagnets are encased in a cooling jacket which forces a cooling liquid through the conductor coils. The jacket has an inner cylindrical wall surrounding the magnet's central bore. It is common to place the shims immediately adjacent to this cylindrical wall. Because the shims must be cooled. they are placed on the same side of the cylindrical wall as the conductor turns. FIG. 1 depicts a standard “Z Shim,” which consists of two loops. The lower view graphically depicts the location of these loops with respect to the coordinate system. Both loops have a given radius “a.”
The upper view in FIG. 1 is an azimuthal plot depicting the Z Shim's coils location in terms of the ratio z/a (which is the location of the coil conductor along the z-axis divided by the radius “a” of the coil). Those skilled in the art will realize that the depiction of FIG. 1 could apply to a magnet of any size. The 0 degree azimuthal position on the plot refers to the x-axis, while the 270 degree position refers to the y-axis.
The arrows in the upper view of FIG. 1 depict the direction of electrical current flow in the two loops. An external voltage is typically applied to the two loops in order to create the current flow and the desired magnetic effects. The reader will observe that the current flows in opposite directions in the two loops. Referring to the lower view of FIG. 1, the current in the upper loop flows in a clockwise direction, while the current in the lower loop flows in an anticlockwise direction. The magnetic field created by these currents lies along the z-axis, with off-axis components being negligible.
FIG. 2 shows a standard “Z2 Shim.” This prior art shim uses four current loops spaced as shown in the view. The current in the top and bottom loops flows in a clockwise direction, while the current in the two middle loops flows in an anticlockwise direction. The magnetic field created lies along the z-axis. Those skilled in the art will know that the field created by the Z2 shim differs significantly from that created by the Z shim. For the Z shim, the field strength varies roughly linearly as one travels from the mid plane through the position of the coils. For the Z2 shim, the field strength varies exponentially. Thus, using both the Z and Z2 shims can be useful in “tuning” the magnetic field.
FIG. 3 shows a standard “X Shim.” It employs two pairs of loops, with one lying above the mid plane and one lying below it. Electrical current in the loops flows as shown by the arrows. The reader will observe that the magnetic field created will lie primarily along the x-axis. Because the loops are not completely orthogonal to the x-axis, there will be some off-axis components. However, since the current in the two pair of loops flows in opposite directions, these off-axis components tend to be cancelled out.
FIG. 4 shows a prior art “Y Shim.” Like the X Shim, it employs two pair of coils, with one lying above the mid-plane and one lying below it. However, the coils of the Y Shim are rotated 90 degrees about the z-axis with respect to the coils of the X Shim. The current flows as shown by the arrows. The magnetic field created lies primarily along the y-axis, with the off-axis components again being cancelled out.
FIG. 5 shows a prior art “XZ Shim.” It also employs two pair of coils located above and below the mid-plane. However the current flow is different than for the X and Y Shims. The current in a given loop lying above the mid-plane is the same as in the corresponding coil lying below the mid-plane (For the X and Y Shims it is opposite). The magnetic components along the y-axis cancel out (or very nearly cancel out), but the components lying along the x-axis and z-axis do not. Hence, this shim is known as an XZ Shim.
FIG. 6 shows a prior art “YZ Shim.” It employs the same coil arrangement as the XZ Shim, except that the coils are rotated 90 degrees about the z-axis. This shim produces a magnetic field having y components and z components, but negligible x components.
Practical shimming configurations combine the shims depicted in FIGS. 1-6, as well as others. The conductive paths must be overlaid around the magnet's core. The various loops overlap at certain points, with insulation being laid between them to prevent a short. FIG. 7 is an azimuthal plot showing the loops of FIGS. 1-6 overlaid. While it is very difficult to discern the individual loops in FIG. 7, the reader may readily appreciate that numerous overlaps exist. The horizontal components of the X shim shown in FIG. 3 and the Y shim shown in FIG. 4 overlay each other to form a double thickness for a large distance. Likewise, the horizontal components of the XZ shim shown in FIG. 5 and the YZ shim shown in FIG. 6 overlay each other for a substantial distance. The vertical components of the X shim of FIG. 3 and the XZ shim of FIG. 5 overlay each other for substantial distances. The vertical components of the Y shim of FIG. 4 and the YZ shim of FIG. 6 overlay each other for substantial distances.
These significant overlays create additional thickness in the shim layer. As stated previously, the shims are typically located on the conductor side of the cooling jacket's inner cylindrical wall. Any added thickness in this region moves the innermost conductor further away from the magnet's central axis, with a consequent reduction in field strength. Thus, it is preferable to provide shimming current loops having a reduced thickness. The present invention achieves this objective.